Center: Dominating Chromatium (a reddish purple sulfur phototrophic bacterium) population from the Lake Cisó (Girona, Spain). Inside Chromatium cells there are deposited elemental sulfur globules. About 107 cells are present per milliliter of lake water. Since Lake Cisó is relatively shallow and protected from winds by a barrier of trees and shrubs which avoids the oxygenation of the top water, for most months in the year sulfide can reach well-lit zones at the surface. Drawing by Christie Lyons (See article by S. Ramírez-Moreno, pp. 235-242, this issue.)

Upper left: Several virions of human adenovirus isolated from the urine of a bone-marrow-transplant patient. Virus particle size is around 75 nm. Negative-staining with potassium phosphotungstate. Transmission electron micrograph courtesy of Laureano Cuevas (Electron Microscopy Unit, National Center for Microbiology, Majadahonda, Spain). (Magnification, ca. 175,000×)

Upper right: Transmission electron micrograph of the bacterium Clostridium bifermentans and its endospore, isolated from Ebro Delta microbial mats. Ultrathin sections were collected on copper grids and poststained with uranyl acetate and lead citrate. Samples were visualized on a JEM1010 TEM microscope (JEOL). This strain was isolated and photographed by Laura Villanueva (Department of Microbiology, University of Barcelona, Spain). (Magnification, ca. 35,000×)

Lower left: Culture of the protist Leishmania infantum promastigotes, kindly provided by F. Grimm (University of Zürich, Switzerland). Photographed after Giemsa-staining by María Colmenares (Molecular Parasitology Laboratory, Biological Research Center, CSIC, Madrid). (Magnification, ca. 2000×)

Lower right: The fungus Syncephalastrum racemosum CECT 2569. Spores develop in rows in elongated merosporangia. Scanning electron micrograph courtesy of Josefa Bosch, Jordi Abellà, and Montserrat Agut (Chemical Institute of Sarrià, Ramon Llull University, Barcelona, Spain). (Magnification, ca. 2000×)